Austin is a pretty fucking casual town. So, if they are specifying “festival casual” as the dress code, they are not expecting black tie. Take a look at pictures of ACL or SXSW to get an idea of what goes, which is pretty much anything.
The main thing to adjust for is the weather, You haven’t said what time of the year this is happening. If it’s Dec through March, I’d probably stick with long pants. In the hotter months, I wouldn’t hesitate to wear shorts. You need the ventilation. I know a guy who showed up to his daughter’s wedding in shorts.
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